1. Field of the Invention
The present invention relates to a method for preparing a noble metal-supported zeolite catalyst for selective reduction of nitrogen oxides exhausted under excess oxygen conditions. In particular, the present invention relates to a method for preparing a noble metal-supported zeolite catalyst effective for catalytic reduction of nitrogen oxides with methane, which comprises of filling the pores of zeolite with organic compounds prior to supporting active noble metal catalyst components on the zeolite. Through this preparation method, it is possible to locate noble metal components, which is essential for the designing of highly active catalysts for reduction of nitrogen oxides, precisely on the desired positions of zeolite pores.
2. Description of the Prior Art
Since Armor et al reported that nitrogen oxides (NOx) could be selectively reduced over a cobalt ion-exchanged Co-ZSM-5 catalyst by using methane as a reducing agent (Y. Li, and J. Armor, Appl. Catal. B 1 (1992) L31), it was recognized that the activation temperature of hydrocarbons is closely related with the temperature window and the activity of the selective reduction (SCR) of NOx. Accordingly, the research to develop noble metal-supported catalysts, for example platinum or palladium-supported catalysts having a high methane oxidation activity under a lean-bum condition for the SCR of nitrogen oxides has been actively carried out (Y. Nishizaka, and M. Misono, Chemistry Letter, 2237 (1994); J. H. Lee, and D. L. Trimm, Fuel Processing Tech., 42 (1995) 339). The said noble metal catalysts are supported on a support such as silica, zirconia, titanium and zeolite but their catalytic performance depends significantly on the types of supports. The catalysts supported on a non-microporous support such as alumina, silica, zirconia and titanium exhibit poor activity in excess oxygen atmosphere where oxygen content is 30% or more, while the catalysts supported on a zeolite having regular size of micropores maintain high SCR activity of NOx in excess oxygen atmosphere (R. Burch, and A. Ramli, Appl. Catal. B, 15 (1998) 49). Accordingly, it is thought that such noble metal-supported zeolite catalysts will become more and more important for the treatment of the exhaust gases from fixed sources excess oxygen such as gas turbines or boilers, or lean-bum engines.
A number of NOx-SCR catalysts wherein catalytic active noble metals such as platinum, palladium and rhodium supported on a zeolite and processes employing said catalysts have been suggested. Kanesaka et al. of Nissan Motor Co., Ltd., disclosed a catalyst comprising first layer of mononith coated with platinum, palladium or rhodium catalyst supported on alumina and second layer coated with copper or cobalt ion-exchanged on ZSM-5, mordenite or ferrierite, which showed excellent catalytic performance for the treatment of exhaust gas from lean-burn engines (U.S. Pat. No. 5,427,989 (1995)). Abe et al. of NGK Insulator, Ltd., tried to increase thermal stability of the noble metal supported zeolite catalysts by mixing them with alumina, titania, zirconia or silica (U.S. Pat. No. 5,164,350 (1992)). In addition, Oshima et al. of Toyota Jidosha Kabushiki Kaisha disclosed a NOx-SCR process based on Pt ion-exchanged zeolite catalyst working in internal combustion engines at the temperature range of 100 to 150xc2x0 C., in which hydrogen was used as a reductant produced from methanol over a Cu-Ni-Cr/alumina reforming catalyt (U.S. Pat. No. 5,412,946 (1995)). Recently, Gardner et al. of Low Emissions Technologies Research and Development Partnership proposed a catalyst comprising a metal hydrate support such as titanium and zirconium doped with platinum, palladium, or a combination of these working in high oxidizing atmosphere and when the said catalyst was modified with an alkali or an alkaline earth metals further improvement brought in catalytic activity (U.S. Pat. No. 5,830,421, (1998)). Hepburn et al. of Ford Global Technology improved NOx-SCR activity by adding a NOx trapping material to a SCR catalyst comprising of Co, Cu, Pt, Au or Ag loaded on a zeolite or a heat resistant oxide. That is, Hepburn et al. succeeded in improving the NOx reduction performance of the catalyst by providing a noble metal-supported porous material to contact with exhaust gas for absorbing NOx prior to be reduced by the reductant (U.S. Pat. No. 5,727,385 (1998)).
As discussed above, since the noble metal supported on non-microporous supports are too simple in their catalytic functions to reduce selectively the NOx in excess oxygen condition, there have been an effort to design hybrid type catalysts comprising of a hydrocarbon oxidation site such as highly dispersed noble metal and a NOx reduction sites such as metal ion-exchanged micorporous zeolite through the introduction of adsorption capability of NOx. The said approaches contributed to increase catalytic activity at low temperature but there still remains further improvement lo bring sulfur resistance to the catalyst.
The objective of this invention is to provide a preparation method of NOx-SCR catalyst of which catalytic activity has increased by two times or more compared to that of conventional catalysts. In the present invention, in order to load noble metal components on a zeolite support, a new supporting method has been adopted different from simple impregnation or ion-exchange methods employed in the prior conventional methods. When the noble metals are supported on a zeolite according to the present invention, the expensive noble metals can be highly and precisely dispersed around the microporous zeolite compared to conventional methods. The prepared supported catalysts exhibit excellent NOx reduction activity more than two tunes higher than that of conventional catalysts especially in excess oxygen atmosphere.
The term xe2x80x9cNOx reduction catalystsxe2x80x9d means the catalysts capable of selectively reducing nitrogen oxides by the use of natural gas as a reducing agent in the presence of excess oxygen. The present invention is based on the concept of bifunctional catalyst proposed by Sang-Eon Park and Misono et al. Namely, the catalyst of the present invention is based on the result that the co-existence of two catalytic components capable of oxidizing hydrocarbons and reducing nitrogen oxides through the interaction with these activated hydrocarbons brought synergistic effect in catalytic activity, thereby making it possible to provide an efficient catalyst for selective reduction of nitrogen oxides (S.-E. Park, React. Kinef. Catal. Lett., 57 (1996) 339; J.-Y. Yan, H. H. Kung, W. M. H. Sachtler and M. C. Kung, J. Catal., 175 (1998) 294; C. Descorme, P. Gelin, C, leuyer and M. Primet. J. Catal., 177 (1998) 352.). Several types of preparation methods have been proposed to combine these two catalytic functions into one. The first is a method of physically mixing a noble metal component having excellent oxidizing activity with ion-exchanged zeolite catalysts having high NOx reduction capability. The second is a method of supporting an excess amount of noble metal catalyst component to a support (Korean patent application No. 96-956; M. Misono, Cattech. June (1998) 53). However, these two methods are not efficient to design a low temperature NOx-SCR catalyst working in excess oxygen condition.
Therefore, the objective of present invention is to provide a preparation method of NOx-SCR catalyst working with methane reductant at low temperature of 400xc2x0 C. or less.
The other objectives and features of the present invention will become apparent to those skilled in the art from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.
The present invention relates to a method of preparing a catalyst for selective reduction of nitrogen oxides using natural gas as a reducing agent in the presence of excess-oxygen, which comprises filling zeolite""s micropores with organic compounds having molecular weight of 100 to 250 prior to loading catalytically active noble metal components on a zeolite.
Since the catalytically active noble metal components could be loaded on the zeolite under the pores of which are filled with organic compounds, the noble metal component essential for hydrocarbon activation can be supported precisely on the desired positions of zeolite""s pores.
The catalyst prepared by this new method exhibits higher NOx reduction activity compared to the catalysts prepared by the conventional impregnation method in which activated noble metal components are loaded on a support micropores of which are not protected by organic compounds. In other words, the catalyst prepared in the present invention is able to reduce NOx selectively by the natural gas as a reducing agent in the presence of excess oxygen with high activity. This prepared catalyst is helpful to remove NOx from exhaust gas containing excessive oxygen such as lean bum engines or stationary sources like gas turbines or boilers.
In particular, the present invention provides a catalyst for selective catalytic reduction of NOx in oxidizing atmosphere by hydrocarbons, which are inexpensive and safe, as a reducing agent. The present invention also provide a highly active NOx-SCR catalyst working with natural gas, which is the most economical, as a reducing agent. Since the natural gas consists of 85% or more of methane which is a very stable hydrocarbon, at least 450xc2x0 C. is required for the natural gas to be used as a reducing agent. In addition, it is known that only limited catalysts such as Pt, Pd, Co and Ga show reaction activity with natural gas.
The said noble metal component can be a mixture of at least one or two selected from the group consisting of transition metals of VIII and IB groups of the periodic table, such as platinum, palladium, rhodium and gold. When two kinds of noble metals are mixed (AxBy), x is preferably 0.5 to 0.99 and y is preferably 0.01 to 0.5. It is particularly preferred that A is palladium or gold and B is rhodium or iridium. The content of active noble metal component supported is 0.3 to 2.0% by weight based on the total catalyst weight under dried conditions.
To protect the micropore of zeolite and locate active metal components near the external site of zeolite, an organic compound having molecular weight of 100xcx9c250 is used such as alcohols or amines having secondary propyl, t-butyl or t-pentyl groups, or salts of amines having secondary propyl, t-butyl or t-pentyl groups with OH, Cl, Br or I, or quinone. The most preferable form of organic compounds is one of tetrapropyl ammonium hydroxide (hereinafter referred to as xe2x80x9cTPAOHxe2x80x9d), tetrapropyl ammonium bromide (hereinafter referred to as xe2x80x9cTPABrxe2x80x9d), tetrabutyl ammonium hydroxide (hereinafter referred to as xe2x80x9cTBAOHxe2x80x9d), tetrabutyl ammonium bromide (hereinafter referred to as xe2x80x9cTBABrxe2x80x9d). These organic compounds are added to the zeolite in the form of solution corresponding to the exact pore volume of zeolite so that 0.5 to 5 mole of organic compounds can be added per mole of Al consisting zeolite skeletal.
Said zeolite is preferably one of H-type zeolite such as BEA, MFI and USY having Si/Al ratio of 10xcx9c100.
The alkaline earth metal or transition metal such as titania, vanadia and ceria can be added further together with said active noble metal components in an amount of 0.5xcx9c5.0% by weight on the basis of total dried catalyst weight. The metal oxides of vanadia or ceria are capable of improving the oxidizing capability of the catalyst; and metal oxides such as titania are capable of increasing the dispersion of the supported noble metal components.
As a method for loading active noble metal components on zeolite micropore of which is filled with organic compounds, an impregnation method and ion-exchange method explained below are particularly preferred.
(1) Impregnation Method
A solution in which noble metal components are dissolved (10xcx9c30 ml/g catalyst), preferably an aqueous solution containing noble metal salts corresponding to a ratio of 5xcx9c10 ml per 1 g of zeolite is poured on a zeolite. The zeolite slurry solution is stirred for 1xcx9c4 hours at 50xcx9c70xc2x0 C. and then the solvent is removed by a vacuum evaporating method, for example, using a rotary vacuum evaporator, thereby supporting catalytically active noble metal component on a zeolite.
(2) Ion-exchange Method
A solution in which noble metal components are dissolved (50xcx9c1000 ml/g catalyst), preferably an aqueous solution containing noble metal salts corresponding to a ratio of 300xcx9c500 ml per 1 g of zeolite is poured on a zeolite. The zeolite slurry solution is stirred for 12xcx9c24 hours for ion-exchange, and then washed and dried. To remove the noble metal salts remaining at the external site of the catalyst, the catalyst after ion-exchange is sufficiently washed with distilled-water in a filter flask.
The supported catalyst is dried at 100xcx9c150xc2x0 C. for 3xcx9c5 hours and then is subjected to be calcined at 400xcx9c600xc2x0 C. for 3xcx9c6 hours in air atmosphere for activation. When the calcination temperature is less than 400xc2x0 C. or the calcination time is too short, the metal components are not dispersed well around the zeolite, thereby deteriorating the activity of NOx reduction. Meanwhile, if the calcination temperature is excessively high over 700xc2x0 C., the structure of the catalyst is destroyed, thereby deteriorating the efficiency of NOx reduction.
In order to prepare catalysts loaded with two or more of catalytic active noble metals, a solution containing two or more of active metals is made, which is then loaded on a zeolite in the same manner as the said impregnation method. The catalysts in which two or more of active metals such as Ptxe2x80x94Pd, Ptxe2x80x94Rh, Pdxe2x80x94Rh or Pdxe2x80x94Ag are loaded on a zeolite revealed activity much higher than that of single metal component. The loading ratio of these two metals are between 0.1 to 0.9 expressed as molar ratio.
The obtained noble metal-supported zeolite catalyst is finally subjected to calcine treatment at 400xcx9c600xc2x0 C. in air atmosphere for its activation.
The present invention is also related with a catalyst for the reduction of nitrogen oxides prepared by the method mentioned above.
The nitrogen oxides can be removed by way of NOx-SCR using the said catalyst prepared by the present invention under the reaction conditions as follows the mixed volume ratio of NO:natural gas: oxygen is 1: (0.1xcx9c10): (10xcx9c1000) and the space velocity is 10,000xcx9c50,000 hxe2x88x921. The preferable ratio of natural gas: oxygen is 1: (20xcx9c100). If the ratio is less than 1, the activity of the catalyst deteriorates. Meanwhile, if the ratio is more than 100, it lacks economic advantage.
As a reducing agent, natural gas having 1 or more carbon atoms or mixtures thereof can be used. The major component of natural gas is methane and this is used as a reducing agent for the reduction of nitrogen oxides.
Generally, for the preparation of NOx-SCR catalysts, a physical mixing method in which catalytically active components having different functions are physically mixed is employed. However, the present invention improves the loading method of active metal components on zeolite through the mixing in molecular level while maintaining functions of each catalytic components.
When the temperature for SCR is 400xc2x0 C. or less, no catalytic activity is observed due to no activation of natural gas. When the temperature is excessively high over 600xc2x0 C., the reducing agent is oxidized with oxygen to form carbon dioxide, thereby deteriorating the SCR activity.
In order to determine the SCR activity, the catalysts were activated at 400xcx9c600xc2x0 C. in an oxygen atmosphere for 3 hours. A mixed gas consisting of NO, natural gas, oxygen and helium was introduced through a mass flow controller at GHSV of 10,000xcx9c50,000 hxe2x88x921 and then the NOx reduction is carried out at 400xcx9c550xc2x0 C. for 100 hours. The conversion of NOx was 85%.
The present invention is also related with a catalyst DeNOx system installed in stationary sources such as gas turbines or boilers or mobile sources for removal of NOx in exhaust gas. This DeNOx catalyst system can be installed on the conventional stationary or mobile sources for the purification of exhaust gas.